For example, an Automatic Transmission (AT) or a Continuously Variable Transmission (CVT) for an automobile has a seal ring that is to be attached to an annular groove provided on the outer peripheral surface side of a shaft and seals the annular gap between the shaft and a housing. In such an art, a shaft with a seal ring attached to its annular groove in advance is inserted in a shaft hole provided in a housing.
Here, there is a case that the seal ring could be damaged when the shaft is inserted in the shaft hole. A reason why the seal ring could be damaged will be described with reference to FIG. 9. FIG. 9 is a view for describing the operation of inserting a shaft in a housing in the related art. As described above, a shaft 200 with a seal ring 500 attached to an annular groove 210 in advance is inserted in a shaft hole 310 provided in a housing 300 in a direction indicated by arrow X in the figure. At this time, the seal ring 500 protrudes downward from the outer peripheral surface of the shaft 200 due to its own weight. Therefore, when a lower part Y of the seal ring 500 abuts against the tip end of the housing 300, there is a case that the lower part Y could be damaged. Note that the seal ring 500 is configured to closely adhere to each of the sidewall surface of the annular groove 210 and the inner peripheral surface of the shaft hole 310 to exhibit its sealing function. Therefore, a gap is formed between the inner peripheral surface of the seal ring 500 and the groove bottom of the annular groove 210. Thus, in the operation of inserting the shaft 200 in the shaft hole 310, the seal ring 500 is in a state of suspending from the annular groove 210 due to its own weight, and a central axis CS of the seal ring 500 is positioned below a central axis C of the shaft 200. Accordingly, the seal ring 500 protrudes downward from the outer peripheral front surface of the shaft 200. Note that dotted lines in FIG. 9 indicate the position of the seal ring 500 in a case in which the central axis CS of the seal ring 500 is coincident with the central axis C of the shaft 200.
Here, the interval between the central axis C of the shaft 200 and the central axis CS of the seal ring 500 in the operation greatly depends on the gap between the inner peripheral surface of the seal ring 500 and the groove bottom of the annular groove 210. Therefore, with a reduction in the gap, it becomes possible to narrow the interval between the central axis C and the central axis CS. However, as shown in FIG. 10, because of manufacturing reasons there is a case that corner parts 213 at both ends of the groove bottom of the annular groove 210 are constituted by a curved surface connecting the groove bottom 212 of the annular groove 210 and a groove side surface (sidewall surface 211) to each other. Note that FIG. 10 is a schematic cross-sectional view of a sealing structure provided with a seal ring 500X according to a conventional example. As described above, the seal ring 500 closely adheres to the sidewall surface 211 of the annular groove 210 to exhibit its sealing function. Therefore, it is necessary to prevent the seal ring 500 from contacting the corner parts 213 constituted by the curved surface. Accordingly, when the corner parts 213 constituted by the curved surface are large, it is necessary to expand the gap between the inner peripheral surface of the seal ring 500 and the groove bottom 212 of the annular groove 210.
As a countermeasure for this problem, it is assumed as shown in FIG. 11 to employ an art in which a seal ring 500Y has concave parts 510 at the respective ends of both side surfaces on its inner peripheral surface side. Note that FIG. 11 is a schematic cross-sectional view of a sealing structure provided with the seal ring 500Y according to a conventional example. However, in the case of a resinous seal ring, an abutment joint part is often provided at one position in a circumferential direction thereof in order to enhance the operability of attaching the seal ring to the annular groove 210. In addition, the abutment joint part may have a complicated structure due to its functions or the like. In such a case, the concave parts 510 are not allowed to be provided near the abutment joint part due to their strength, manufacturing processes, or the like.
Note that various materials such as PTFE and PEEK are used for the seal ring, but the PTFE is preferably used in a portion requiring sealing performance. Since the PTFE is relatively soft, the problem in which the seal ring could be damaged becomes more noticeable. In addition, in the case of an AT or a CVT for an automobile, it is possible to confirm damage on a seal ring by examination after the AT or the CVT is assembled. Therefore, the operation of replacing the seal ring becomes very troublesome.